1. The Field of the Invention
The present invention relates to a femoral hip prosthesis for replacing a portion of proximal femoral bone during hip replacement and the methods of assembly and use thereof.
2. The Relevant Technology
Total hip arthroplasty using a metallic hip prosthesis has been successfully performed since the early 1960's and is now a routine procedure to address orthopedic diseases such as osteoarthritis, fracture, dislocations, rheumatic arthritis, and aseptic or avascular bone necrosis. During this procedure, the bone is prepared for the prosthesis by removing the damaged articulating end of the bone by resecting a portion of the bone including the femoral head. This exposes the inside, of the metaphaseal region of the intramedullary canal in the proximal femur. The surgeon then drills or reams a cavity in the femur approximately in line with the intramedullary canal. This cavity is used to align other tools such as reamers, broaches and other bone tissue removal instruments to create a roughly funnel shaped bone cavity that is smaller in cross-section as it extends down from the bone resection at the proximal end of the femur into the distal intramedullary canal. This funnel shaped cavity is typically also eccentric with more bone material removed from the medial calcar region of the proximal femur than the region on the lateral side of the canal.
Oftentimes a grouting agent commonly referred to as bone cement is then added to the funnel shaped cavity. Once the prosthesis is inserted into the cavity, this creates a bone cement mantle between the prosthesis and the bone. Sometimes the shape of the cavity is prepared to closely match the shape of the external surface of the prosthesis, and the prosthesis is press fit into the cavity without the use of bone cement. These press-fit prostheses typically have a textured bone-ingrowth surfaces place strategically at specific locations on their surface to help facilitate lone-term bone tissue growth into the prosthesis. This bone ingrowth into the porous structure on the implant creates a long lasting secure bond between the prosthesis and the proximal femur.
Once the bone cavity is prepared, the prosthesis is placed into the bone cavity and is supported directly by internal bone tissue in the case of a press fit implant or indirectly by the bone cement mantle in the case of the cemented implant. Then, the prosthesis is aligned such that the articulating end of the implant articulates with the opposite side of the natural joint in the case of a hemiarthoplasty, or articulates with a corresponding implant replacing the opposite side of the joint in the case of a total joint arthroplasty.
Current designs of proximal femur hip prosthesis have eccentric, non-symmetric cone shaped central body portions. The current methods of implant fixation allow for transfer of axial loads to the proximal femur mainly through shear stresses at the eccentric funnel shaped bone-prosthesis interface. The effective transfer of load is significantly dependent on the three-dimensional shape of funnel shaped cavity, the bone-prosthesis or bone-cement-prosthesis interface as well as physiological loading of the proximal end. Partly because of the eccentrically shaped cross-section of the central body portion, these currently available prostheses transmit radial expansion forces on the proximal femoral cavity as the implant is loaded in compression. The funnel shape of the cavity and the matching shape of the implant or bone cement result in circumferential hoop stresses and radial expansion stresses are distributed to the bone as the femoral component is axially loaded. This results in complex axial and shear stresses at the bone-implant interface. Consequently, the distribution of the loads that transmit from the femoral head axially through the proximal femur is altered after THA.
A potential cause of failure of currently used prosthesis is associated with the possible resorption of the bone surrounding the implant. The bone resorption can be the result of an altered distribution of shear stresses on the remaining proximal femoral tissue. In time, the lack of adequate stress transfer from the metal stem to the surrounding bone may cause a loss of bone density, resulting in the increased possibility of bone failure or loosening of the bone-prosthesis interface. The gradual loss of bone support in the calcar region of the eccentric cavity increases the bending load that must be borne by the prosthesis. This increase in bending load on the prosthesis can lead to stress shielding by the prosthesis resulting in prosthesis fatigue and potentially to eventual clinical failure.